Praise be to Allah, the Lord of the Worlds.
Preaching after congregational prayer isn`t considered an act of disturbance for several reasons among which are the following:
First: Most of the time, the Prophet (PBUH) used to preach after finishing the congregational prayer, and it was never narrated that he (PBUH) waited for late comers to finish their prayer nor for others to finish Dhikr (Remembrance of Allah). A proof of this is the following Hadith: "Narrated Zaid bin Khalid Al-Juhani: Allah's Messenger (PBUH) led the Morning Prayer in Al-Hudaibiya and it had rained the previous night. When the Prophet (PBUH) had finished the prayer, he faced the people and said: "Do you know what your Lord has said?" They replied: "Allah and His Apostle know better." (The Prophet (PBUH) said): "Allah Says (What means): "In this morning some of My worshipers remained as true believers and some became non-believers; he who said that it had rained with the blessing and mercy of Allah is the one who believes in Me and does not believe in star, but he who said it had rained because of such and such (star) is a disbeliever in Me and is a believer in star." {Related by Bukhari & Muslim}.
This is in addition to many other Hadiths underlining the same idea. Rather, it was confirmed in [Sahih Al-Bukhari] that the Prophet (PBUH) used to make Dhikr loudly, sometimes, after finishing the congregational prayer and didn`t refrain from doing so for fear of disturbing those who came to prayer late.
Second: A preacher wouldn`t force anyone to listen, so one who doesn`t feel like listening is at liberty to go to one end of the mosque to continue Dhikr or recitation. Rather, the Hanafi Madhab recommends standing up directly after finishing prayer and making Dhikr at home after delivering the Sunnah prayer. This was stated in the book [Raad Al-Mohtaar, vol.1/pp.531].
Third: Taking a closer look at the Hadiths that prohibit disturbing prayer performers shows that they focus on raising the voice, while reciting the Quran, in an exaggerated manner that leads to disturbing others. This is reflected in the following Hadith: " Yahya related to me from Malik from Yahya ibn Said from Muhammad ibn Ibrahim ibn al Harith at-Taymi from Abu Hazim At-Tammar from al Bayadi that the Messenger of Allah, may Allah bless him and grant him peace, came out to the people while they were praying and their voices were raised in the recitation. He said: "When you pray you are talking confidentially to your Lord. So look to what you confide to Him, and do not say the Qur'an out loud so that others hear it." {Related by Malik in "Al-Mowatt`a"}.
Fourth, the time after the congregational prayer is suitable for preaching on many events where the preacher would remind the people with the ruling on frequently asked questions, command them to obey Allah and avoid violating the teachings of Allah.This is why the Prophet (PBUH) used to utilize that particular space of time a lot. Therefore, prayer performers shouldn`t get annoyed nor deny the Imam`s preaching.
Five: The preaching shouldn`t exceed a few minutes. The preacher should deliver a clear brief message to avoid causing boredom to the prayer performers. Narrated Abu Wail: "Abdullah used to give a religious talk to the people on every Thursday. Once a man said: "O Aba `Abdur-Rahman! (By Allah) I wish if you could preach us daily." He replied: "The only thing which prevents me from doing so, is that I hate to bore you, and no doubt I take care of you in preaching by selecting a suitable time just as the Prophet (PBUH) used to do with us, for fear of making us bored." {Related by Bukhari & Muslim}. In conclusion, it is worth reminding that the preacher should ask for the permission of the permanent Imam (Appointed by the Ministry of Awqaf) before delivering the sermon so as to avoid creating an argument inside the mosque. And Allah Knows Best.